Segment parsing and embedded PHP errors

So basically we need to check the pattern of a segment and if it doesn't match our patterns we go on to start our entries loop with a category (via seg2cat).

if {segment_2} is a date, it's either formatted YYYY-MM or YYYY-MM-DD, otherwise it's a string.

<?php 

    $this->EE =& get_instance();
    $seg2 = $this->EE->uri->segment(2);

    $date_pattern_half = '/^[0-9]{4}-(0[1-9]|1[0-2])$/';
    $date_pattern_full = '/^[0-9]{4}-(0[1-9]|1[0-2])-(0[1-9]|[1-2][0-9]|3[0-1])$/';

    if( preg_match($date_pattern_full, $seg2) || preg_match($date_pattern_half, $seg2) ) {
?>
        {exp:low_events:entries channel="calendar" date="{segment_2}"}
<?php } else { ?>
        {exp:low_events:entries channel="calendar" category="{segment_2_category_id}"}
<?php } ?>

... do stuff with entries here ...

{/exp:low_events:entries}

The error we're getting back is:

Parse error: syntax error, unexpected '}' in /server/domain/system/expressionengine/libraries/Functions.php(679) : eval()'d code on line 68

for reference, the above function in Functions.php is:

/**
 * eval()
 *
 * Evaluates a string as PHP
 *
 * @access  public
 * @param   string
 * @return  mixed
 */
function evaluate($str)
{
    return eval('?'.'>'.$str.'<?php ');
}

As for the 'line 68' val it's just a closing </ul> tag.

I should also note that when we remove the opening loops ({exp:low_events:entries}) we don't get this error.

We've tried the alternative syntax <?php else: ?> etc but we get an unexpected T_ELSE instead of the one above.